Electrical band-pass filters are used in many different types of consumer and industrial electronic product to select or reject electrical signals in a range of frequencies. In recent years, the physical size of such products has tended to decrease significantly while the circuit complexity of the products has tended to increase. Consequently, a need for highly miniaturized, high-performance band-pass filters exists. A special need for such band-pass filters exists in cellular telephones in which the antenna is connected to the output of the transmitter and the input of the receiver through a duplexer that includes two band-pass filters.
Modem cellular telephones incorporate a duplexer in which each of the band-pass filters includes a ladder circuit in which each element of the ladder circuit is a film bulk acoustic resonator (FBAR). Such a duplexer is disclosed by Bradley et al. in U.S. Pat. No. 6,262,637 entitled Duplexer Incorporating Thin-film Bulk Acoustic Resonators (FBARs), assigned to the assignee of this disclosure and incorporated into this disclosure by reference. Such duplexer is composed of a transmitter band-pass filter connected in series between the output of the transmitter and the antenna and a receiver band-pass filter connected in series with 90° phase-shifter between the antenna and the input of the receiver. The center frequencies of the pass-bands of the transmitter band-pass filter and the receiver band-pass filter are offset from one another.
FIG. 1 shows an exemplary embodiment of an FBAR-based band-pass filter 10 suitable for use as the transmitter band-pass filter of a duplexer. The transmitter band-pass filter is composed of series FBARs 12 and shunt FBARs 14 connected in a ladder circuit. Series FBARs 12 have a higher resonant frequency than shunt FBARs 14.
FBARs are disclosed by Ruby et al. in U.S. Pat. No. 5,587,620 entitled Tunable Thin Film Acoustic Resonators and Method of Making Same, now assigned to the assignee of this disclosure and incorporated into this disclosure by reference. FIG. 2 shows an exemplary embodiment 20 of an FBAR. FBAR 20 is composed a pair of electrodes 24 and 26 and a layer of piezoelectric material 22 sandwiched between the electrodes. The piezoelectric material and electrodes are suspended over a cavity 28 defined in a substrate 30. This way of suspending the FBAR allows the FBAR to resonate mechanically in response to an electrical signal applied between the electrodes. Other suspension schemes that allow the FBAR to resonate mechanically are possible.
Also disclosed in the above-mentioned U.S. Pat. No. 5,587,620 is a stacked thin-film bulk acoustic resonator (SBAR). FIG. 3 shows an exemplary embodiment 40 of the SBAR disclosed in U.S. Pat. No. 5,587,620. SBAR 40 is composed of two layers 22, 42 of piezoelectric material interleaved with three electrodes 24, 26, 44. An input electrical signal is applied between electrodes 44 and 26 and an output electrical signal is provided between electrodes 24 and 26. The center electrode 26 is common to both the input and the output.
The SBAR disclosed in U.S. Pat. No. 5,587,620 was thought to have promise for use as a band-pass filter because it has an inherent band-pass characteristic. However, practical examples of the SBAR exhibit an extremely narrow pass bandwidth that makes the SBAR unsuitable for use in most band-pass filtering applications, including the cellular telephone duplexer application referred to above. The narrow pass bandwidth of the SBAR can be seen in FIG. 4, which compares the frequency response of a practical example of SBAR 40 shown in FIG. 3 (curve 46) with the frequency response a practical example of the FBAR-based band-pass ladder filter shown in FIG. 1 (curve 48). FIG. 4 also shows that, while the frequency response of the ladder filter shown in FIG. 1 advantageously falls sharply outside the pass-band, as the frequency difference from the center frequency further increases, the frequency response undesirably rises again.
What is needed, therefore, is a band-pass filter with a low insertion loss and flat frequency response in its pass band, a pass bandwidth in the range from about 3% to about 5% of a center frequency anywhere from about 0.5 GHz to about 10 GHz and good out-of-band rejection. What is also needed is such a band-pass filter with the structural simplicity of the SBAR.